Getting pynodes geometric node to work
The version of geometry_nodes as of commit edbdbc8fea539 does a small check that isn't actually really used yet. This looks to be part of a work-in-progress that is as of yet uncompleted. To get these nodes running you'll have to modify modifiers.py and comment out or remove lines 46 and 47:

#if not isinstance(datatype, TypeSpec):
#    raise TypeError("Expected TypeSpec datatype")


After this (re-)start blender, enable the addon and play with the geometry nodes :)

Note that as of this writing the above mentioned commit is the latest and the only one in 24 days. Looks very much like current development is on hold.

A small screenshot of geometry nodes in the node editor: